Biodiversity and genetic relatedness of internal parasites of coral reef fishes collected under the CReefs program

By recent estimates, parasites represent at least 30% of the total animal diversity but are a rarely-seen, cryptic component of the coral reef ecosystem. The fish fauna of Australia's coral reefs is one of the most diverse in the world with a recorded 1,610 described species, of which many occur both on Queensland's Great Barrier Reef and the fringing reefs off Western Australia. In these fish among the most commonly encountered internal parasites are the trematode flatworms, normally found in the gut lumen, and the myxozoans, originally classified as protozoans but now recognised to have affinities with cnidarians and found principally in the muscle, brain and gall bladder of their hosts.

In addition to their significance as part of the normal symbiotic fauna of marine animals, many species of Myxozoa cause mortality in fish, appear as white cysts in fish muscle that render fillets unmarketable, and produce post-mortem fish muscle liquefaction. These infections pose significant challenges in the commercial sector and are also of concern for Australia's biosecurity.

Tuna fillet from Ningaloo Reef, Western Australia showing a high intensity infection of myxozoan (Multivalvulida: Kudoidae) cysts in the muscle.

Taxonomic studies of myxozoans at QM have revealed a diversity that is likely to exceed that of their fish hosts. Analysis of a dataset generated on the GBR over the last 4 years predicts a diversity in one genus, Ceratomyxa, of almost 1 species of parasite for every species of fish. This estimate implies that Australia's coral reef fish fauna harbours over 1,500 species of Ceratomyxa, of which only 20 (or less than 1%) have been formally described. Ceratomyxa is only one genus in the Class Bivalvulida and recent parallel studies based at the Queensland Museum have shown that members of the Class Multivalvulida are also abundant in coral reef fishes. Thus, this group is exceptionally rich in coral reef fishes but exceptionally poorly known. Knowledge of the taxonomy, host-specificity and geographical distribution of these parasites will help understand patterns of infection and pathogenicity of myxosporeans when they interact with wild harvest and aquaculture fisheries.

Line drawings of spores of 10 new species of Ceratomyxa infecting the gall bladder of coral reef fish of the Great Barrier Reef. Scale bar = 10 µm (from Gunter & Adlard, 2009 International Journal for Parasitology.)Spores of the myxozoan parasite from a cyst in the muscle of a tuna from Ningaloo Reef, Western Australia. Each cyst in the fillet pictured previously contains numerous spores with 4 polar capsules. Spore approximately 10µm in diameter.

Trematodes are flatworm parasites that infect all classes of vertebrates but are at their richest in marine fishes. They have complex life cycles involving two, three or four separate hosts and can infect almost any tissue of their vertebrate definitive hosts. Trematodes range from being relatively benign to highly pathogenic; they typically sterilise their molluscan intermediate hosts and the blood flukes can cause significant losses in aquaculture. In contrast to the Myxozoa, the trematodes of Australian coral reef studies have been the subject of sustained study led by Cribb at UQ, since the late 1980s. There are now 230 trematode species and 560 host-parasite combinations known for this region. Despite this progress perhaps only a little more than 10% of the fauna has been characterised, as current estimates suggest that as many as 2000 trematode species may infect Australian coral reef fishes in total.

Trematode flatworm from the gut of a coral reef fish.

For comprehensive analysis of the internal parasites of fish, tapeworms (cestodes) and roundworms (nematodes) parasitic in hosts collected during this project will also be preserved and then scientifically documented.

In this study we intend to sample families of fishes that have received little previous research attention so as to maximise the discovery of new species. We will emphasise fish species that occur both in the Western Pacific (GBR) and Indian (Ningaloo) Ocean sites that the CReefs program targets to allow for comparison of parasite fauna from geographically separated biological regions. We will combine our expertise on these groups of marine parasites to: